LEAVES. 51 



Midrib. 



The elements of which the midrib and lateral veins consist 

 are always to be found in powdered leaves, and may sometimes 

 be turned to account in identifying a leaf, although the struc- 

 ture of these tissues presents general features common to most 

 leaves. 



Next to the epidermis there is usually to be found a layer of 

 collenchymatous cells of varying extent passing into the normal 

 cortical parenchyma (or ground tissue) of the midrib. In the 

 powdered leaves the collenchymatous cells usually exhibit their 

 length, and are easily distinguished by their elongated shape and 

 thickened walls. 



The cells of the cortical parenchyma, on the other hand, in 

 surface view have thin walls, and are rounded or polygonal in 

 herbaceous plants, but usually rectangular or elongated in the 

 leaves of shrubs ; they are usually to be detected in the powder. 



The wood of the midrib and lateral veins may contain vessels 

 tracheids, etc., the size of which is sometimes important; the 

 bast seldom offers valuable indications, but the presence or 

 absence of well-developed pericyclic fibres may often be of use 

 in identifying a powder. 



The anatomical elements that may be found in leaf-powders 

 have been shown to be very numerous, and most of them possesa 

 a certain diagnostic value, either high or low. They afford the 

 pharmacist a ready means both of identifying the powders and. 

 of detecting sophistication. 



